1. Field of the Invention
This invention relates to a liquid fuel vaporizing apparatus for producing vaporized fuel by vaporizing liquid fuel used for a combustion unit.
2. Description of the Prior Art
A liquid fuel vaporizing apparatus shown in FIG. 2 has been disclosed heretofore as the one incorporated in the combustion unit. In FIG. 2, a liquid fuel vaporizing apparatus 20 has a construction wherein a heating plug 10, which is a glow plug for vaporization or the like, is incorporated in a hollow vaporizing pipe made of metal, that is, a tubular body 2. On the outer peripheral surface of the tubular body 2, a number of heat-receiving fins 5 are formed. These heat-receiving fins 5 are formed in the circumferential direction of the tubular body 2. The tubular body 2 is fitted with a vaporized fuel injection pipe 6 in one end part thereof and with a liquid fuel feed pipe 9 in the other end part thereof. In the tubular body 2, the heating plug 10, which is a solid heater core, i.e. the glow plug, is so inserted as to form a fuel passage 8. The heating plug 10 is formed of a member of ceramic or silicon nitride or the like, and it has a heating part wherein a resistance wire (omitted in the figure) made up of tungsten is buried or printed in adhesion. In the downstream part of the fuel passage 8, a fuel outlet part 15 is formed. A liquid fuel introduction port 14 of the liquid fuel feed pipe 9 communicates with the fuel passage 8. In the base end part of the heating plug 10, a terminal 11 is provided. An injection port 7 of the vaporized fuel injection pipe 6 is so provided as to open in a combustion chamber of an ordinary combustion unit. In the FIG. (2), the arrows show the direction of flow of the fuel.
A liquid fuel vaporizing apparatus disclosed in Japanese Patent Laid-Open No. 65213/1988 can be cited as a usual one. Said liquid fuel vaporizing apparatus is characterized in that an inner tube having heat-releasing fins provided on the inner peripheral surface is pressed in and fitted to a tubular body having heat-receiving fins provided on the outer peripheral surface, a heating plug being so inserted into said inner tube as to form a fuel passage, a vaporized fuel injection pipe is fitted to one end of the tubular body, and a fuel introduction pipe is fitted to the other end of the tubular body. In this liquid fuel vaporizing apparatus the heat-releasing fins formed on the inner peripheral surface of said inner tube extend in the longitudinal direction of the inner tube, while the inner tube is constructed of a sleeve made of metal and is disposed in a combustion chamber of a combustion unit. The heating plug functions as a glow plug for vaporization in the combustion unit.
A combustion unit disclosed in Japanese Patent Laid-Open No. 62108/1987, and is provided with a combustion tube and a fuel vaporizing apparatus having a nozzle for injecting vaporized fuel, which opens inside of the combustion tube, and a mesh-shaped body surrounding a heating part of the glow plug for vaporization of the fuel vaporizing apparatus is disposed in a cylindrical part into which the heating part is inserted.
It cannot be said that the liquid fuel vaporizing apparatus used in said combustion unit vaporizes liquid fuel sufficiently, although it can be vaporized rapidly to some degree. The liquid fuel cannot be vaporized satisfactorily as liquid fuel of low grade by the radiant heat of the combustion chamber. The liquid fuel vaporizing apparatus is so designed that the heating plug is normally turned off on the occasion of regular combustion of the combustion unit and the heat energy in the combustion chamber is conducted to the fuel passage in the tubular body from heat-receiving fins formed on the outer peripheral surface of the tubular body so as to vaporize the liquid fuel to produce vaporized fuel, which is jetted into the combustion chamber from the fuel injection pipe.
In the liquid fuel vaporizing apparatus, heat conduction in the tubular body is effected in a mode a boiling heat conduction of two-phase flow of vapor and liquid phases, and when reception of the heat from the inside of the combustion chamber of the combustion unit is not uniform, abrupt boiling on the wall surface, i.e. bumping occurs wherein boiling occurs abruptly only at a higher temperature than the boiling point when a liquid is heated in a local part inside of the tube. Consequently, the fuel takes the liquid phase in the central part of a singlehole passage of the fuel injection pipe, while taking the state of the vapor phase in the peripheral part of the wall surface of the single-hole passage. When the heating plug disposed in the tubular body of the liquid fuel vaporizing apparatus is turned off, a liquid film of the liquid fuel is formed on the surface of said heating plug and the liquid film cannot be vaporized. Besides, the liquid fuel flows through the easiest-to-pass part of the passage and is jetted from the fuel injection without being vaporized, and in some cases, oil droplets are jetted in a bumping state from the fuel injection pipe. Consequently, heat conduction of the heat energy to the flowing liquid fuel is insufficient and the vaporizable property of the liquid fuel is deteriorated, and is undesirable for combustion and causes deposition of carbon inside the tubular body.